As set forth in the related copending Ser. No. 250,373, there is a substantial body of prior art patents concerned with tin or tin alloy electroplating baths and processes for utilizing the same. Some of the more relevant patents for the present purposes include U.S. Pat. Nos. 3,730,853 (Sedlacek et al.); 3,749,649 (Valayil); 3,769,182 (Beckwith et al.); 3,785,939 (Hsu); 3,850,765 (Karustis, Jr. et al.) 3,875,029 (Rosenberg et al.); 3,905,878 (Dohi et al.) 3,926,749 (Passal); 3,954,573 (Dahlgren et al.); 3,956,123 (Rosenberg et al.); 3,977,949 (Rosenberg); 4,000,047 (Ostrow et al.); 4,135,991 (Canaris et al.); 4,118,289 (Hsu); and British Pat. Nos. 1,351,875 and 1,408,148.
It is known from the prior art, as represented by the patents referred to above that tin sulfate and tin fluoroborate are generally employed as sources of the divalent tin bath component, whereas the electrolyte is selected from either sulfuric acid or fluoroboric acid.
In many applications, sulfuric acid, as the electrolyte or acid matrix, would be less corrosive than fluoroboric acid. Thus, from a commercial standpoint, it would be desirable to have available a bright, high speed tin electroplating solution which utilizes sulfuric acid rather than fluoroboric acid. It has been found, however, that when sulfuric acid is used, there is poor anode corrosion and undesirable polarization and current drop result.
Moreover, because it is very time consuming to dissolve tin sulfate in the bath, formulation of the initial bath and its replenishment during use would be greatly simplified if tin fluoroborate were used as the source of divalent tin, instead of tin sulfate. It has been found, however, that such use of tin fluoroborate, with the sulfuric acid electrolyte, compounds the problem of poor anode corrosion and its resulting undesirable effects.